In general, a photoconduction process which utilizes an electrophotographic photoreceptor, comprises the steps of:
(1) generating electric charges by light-exposure; and
(2) transporting the electric charges.
Photoreceptor can be divided into a group in which the steps (1) and (2) above are performed by the same substance, and a group in which they are performed by different substances. A typical example of the former group is a selenium photoreceptor. For the latter group, a combination of amorphous selenium and poly-N-vinyl carbazole is well known. Photoreceptors falling within the latter group have advantages in that a wide range of starting materials can be used in the preparation of the photoreceptors. This may make it possible to increase electrophotographic characteristics such as the sensitivity of photoreceptor and receiving potential, and in that substances suitable for increasing these characteristics can be chosen from a wide range.
Photoconductive materials which have heretofore been used in photoreceptors to be used according to the electrophotographic system include inorganic substances such as selenium, cadmium sulfide, and zinc oxide.
The electrophotographic process disclosed by Carlson in U.S. Pat. No. 2,297,691, uses a photoconductive material comprising a support coated with a substance which is insulative in a dark place and changes its electric resistance depending on the amount of light irradiated during imagewise exposure. In general, the photo-conductive material, after being subjected to dark conditioning for a suitable period of time, is uniformly provided with electric charges on the surface thereof in a dark place. Thereafter, the material is exposed imagewise according to a pattern of irradiation having the effect of reducing surface electric charges depending on the relative energy contained in various parts of the pattern. The surface electric charges thus allowed to remain on the photoconductive substance layer (photoreceptive layer), i.e., an electrostatic latent image, is converted into a visible image by bringing the photoconductive substance layer into contact with suitable detection-indicating substances, i.e., toners.
These toners can be drawn to the surface of the photoreceptive layer according to an electric charge pattern even though they are contained in an insulative liquid or in dry carriers. The thus-drawn toners can be fixed by known techniques such as application of heat, pressure or solvent vapor. Furthermore, the electrostatic latent image can be transferred to a second support. Similarly, the electrostatic latent image can be transferred to a second support (e.g., paper and a film) where it is developed. Electrophotography which is so designed as to form images in the above-described manner is one of image-forming processes.
Some of a fundamental characteristics required for the photoreceptor for use in the electrophotographic process are:
(1) It can be charged at a suitable potential in a dark place;
(2) Little or no dissipation of electric charges occurs in a dark place; and
(3) Irradiation with light permits rapid dissipation of electric charges.
The above described inorganic substances which have heretofore been used as photoconductive materials suffer from various disadvantages although they have many advantages. For example, selenium that is presently in widespread use sufficiently meets the above requirements (1) to (3). However, use of this material is not desirable because production conditions are severe, production costs are high, flexibility is poor, it is difficult to shape into a belt-like form. Furthermore, care must be taken in handling since it is of low resistance against heat or mechanical impact. Cadmium sulfide and zinc oxide are used as photoreceptors in which they are dispersed in resins as binders. However, they cannot be used repeatedly as such since they are inferior in mechanical properties such as smoothness, hardness, tensile strength, and frictional resistance.
In recent years, to remove the above described defects of the inorganic substances, electrophotographic photoreceptors prepared using various organic substances have been proposed, and some of the photoreceptors have been put into practical use. Examples include a light-sensitive material containing poly-N-vinyl carbazole and 2,4,7-trinitrofluorene-9-on as described in U.S. Pat. No. 3,484,237, a light-sensitive material containing poly-N-vinyl carbazole which is sensitized with pyrylium salt-based dye, as described in Japanese Patent Publication No. 25658/73, a photoreceptor containing organic pigment as a major component as described in Japanese Patent Application (OPI) No. 37543/72 (the term "OPI" is used herein to refer to a "published unexamined Japanese Patent application"), and a photoreceptor containing an eutectic complex of a dye and a resin, as described in Japanese Patent Application (OPI) No. 10735/72. Although these photoreceptors are believed to have excellent characteristics and to be of high practical value, in practice, no photoreceptor sufficiently satisfies all the requirements for a photoreceptor, e.g., convenient to produce, exhibits satisfactory electrophotographic characteristics, and a good wavelength selectivity which is required when the photoreceptor is used as a laser beam printer or an indication element.